1. Field of the Invention
The present invention relates to image sensor arrays. More particularly, the present invention relates to CMOS image sensor arrays for still camera applications and to methods for operating those arrays.
2. The Prior Art
Integrated image sensors are known in the art. Such sensors have been fabricated from charge-coupled devices (CCDs) and as bipolar and MOS image sensors.
In the CCD imager art, on-chip frame storage capability has been employed previously. It has been motivated by the need to shift sensed charges out during a video frame time without letting them be contaminated by further exposure as the charges travel across the CCD array. Two storage techniques are commonly employed in the CCD imager art. According to the first technique, a second separate on-chip CCD array is provided under a light shield, and the entire image is quickly shifted along one dimension into the storage array, since shifting in one dimension is fast enough to avoid significant contamination. According to the second technique, line-storage CCDs are provided between the lines of sensors, with local light shielding. In the CCD art, techniques have been developed for preventing leakage and contamination due to minority carrier diffusion and leakage, but these techniques are not applicable to the CMOS sensor array because the silicon fabrication processes are different.
In still-camera applications with randomly-addressable CMOS active-pixel sensors, the problem of how to implement a short-exposure interval with a long readout interval exists. A typical active-pixel area-array image sensor is disclosed in Hurwitz et al., xe2x80x9cAn 800K-Pixel Color CMOS Sensor For Consumer Still Camerasxe2x80x9d, SPIE Vol. 3019, pp 115-124 and comprises a plurality of rows and columns of pixel sensors. The most common method of exposure for this type of sensor array is to cyclicly scroll through the rows so that the integration duration for each row is the same, but can be shorter than the total readout interval. This method of exposure control is known as an electronic shutter.
There are two problems with this type of electronic shutter. First, since each row scans a different time interval, there will be motion artifacts (the shape of moving objects will be distorted). In addition, this scheme requires a very high conversion rate analog-to-digital converter (ADC) implementation. For example, if the array has 1 million pixels, and the readout duration is 1/100 sec (about the maximum acceptable for a hand-held camera), the required conversion rate is 100 million samples/sec. Since the state of the art for commercial ADCs with the required accuracy (10 bits) is about 20 million samples/sec, this means that a total of 5 ADCs would have to be used to allow for 1/100 sec exposures.
Another solution to the exposure problem is to provide a mechanical shutter for the camera. In this mode of operation, the entire array is first reset simultaneously. Then the shutter is opened for the duration of the exposure. After the shutter is closed, readout out can take place a relatively slow rate limited only by the dark current error in the pixels. However mechanical shutters add to the cost and complexity of the camera, and also contribute to camera shake.
Prior art in CMOS storage pixels has not yet addressed the problem of leakage and contamination, even though the problem has been noted. In the paper Yadid-Pecht et al., xe2x80x9cA Random Access Photodiode Array for Intelligent Image Capturexe2x80x9d, IEEE Trans. Electron Devices vol. 38 no. Aug. 8, 1991 pp 1772-1779, a prior-art storage pixel is described. The imager disclosed therein is motivated by the need to access pixel values in random order for certain processing functions, as opposed to being motivated by the need to have a readout interval longer than the exposure interval in high resolution still photography. Yadid-Pecht et al. describe the problems of xe2x80x9ccrosstalkxe2x80x9d and xe2x80x9cleakagexe2x80x9d being xe2x80x9cmuch stronger than predicted,xe2x80x9d but they do not offer any specific ideas on how to ameliorate these problems.
The pixel layout in this prior-art imager shows that the authors did not find a strategy for protecting the sensitive storage node from stray carrier diffusion, nor from light. They reference three papers from the CCD art for xe2x80x9ctechnological solutionsxe2x80x9d to these problems, but the CCD art referenced does not obviously apply to the problem of CMOS storage pixels.
It is therefore an object of the present invention to provide a pixel sensor and an array of pixel sensors which overcome some of the shortcomings of the prior art.
A further object of the present invention is to provide a storage-pixel sensor and an imaging array of storage-pixel sensors which overcomes the scanout problems inherent in prior-art imaging arrays.
Another object of the present invention is to provide an improved electronic shutter method for use with storage-pixel sensors.
Yet another object of the present invention is to provide a storage-pixel sensor and an imaging array of storage-pixel sensors which are compatible with the electronic shutter method of the present invention.
According to one aspect of the present invention, a storage-pixel sensor and an array of storage-pixel sensors suitable for use in an active-pixel area-array image sensor employing an electronic shutter method are disclosed.
According to a second aspect of the present invention, a method for implementing an electronic shutter having a greatly reduced ADC sample rate requirement is disclosed. The electronic shutter mechanism of the present invention has fewer motion artifacts than prior-art electronic shutter implementations.